Media Access Control (MAC) is a widely-accepted protocol for wireless communication systems that is designed to allow a plurality of communication devices to simultaneously access shared-medium. In some wireless systems, such as the IEEE 802.11, MAC employs Carrier Sense Multiple Access With Collision Avoidance (CSMA/CA) to minimize packet collisions.
In the CSMA/CA system, each station (STA) is required to listen to the channel to see whether the channel is busy or idle before sending its packets, wherein the STA is allowed to send its packets only when the channel is determined to be idle. Otherwise, a backoff mechanism is invoked. That is, if the channel is determined not to idle, i.e., busy, or there is a packet collision on the channel, then the STA sets a backoff value in its backoff counter that is randomly selected from a contention window. The backoff value is decremented by one time slot and the STA is allowed to retransmit its prior packets when the backoff value is decremented to zero.
However, there is a possibility that a plurality of STAs set a same backoff period in each backoff counter, which results in packet collisions on the channel. To reduce the possibility of continuous packet collisions, CSMA/CA is designed to double the earlier contention window every time a packet collision occurs. Thus, the contention window is increased in a exponential manner every time a packet retransmission is unsuccessful, which can degrade the throughput of the system because stations have to serve a longer backoff period even if unsuccessful transmission occurs due to a channel error.